This invention relates to a device and a process for shirring tubular films.
Tubular films of collagen or cellulose are used extensively as sausage casings. Thin sausages, such as frankfurters or (German) frying sausages are at the present time mostly filled by machine, by closing a sufficiently long piece of tubular film at one end and pushing its open end over the filling nozzle of the filling machine. For these purposes, the manufacturers market the sausage casings in a convenient form for the user, namely as so-called "caterpillars", which consist of a long piece of tube which has been shirred so that its length is 30 to 40 cm. Such a caterpillar is taken out of its container by the user, closed at one end and held with the other, open, end over the filling nozzle of the filling machine. The filling pressure opens up the shirring and brings the full length of the tube into use.
Usually, the tubular film is shirred by pressing the tube over a fixed shirring mandrel against a solid stop, by the action of toothed shirring rollers engaging on the periphery of the tube. In this process, it is desirable to have the closest possible density of shirring so as to include as much tube as possible in a caterpillar of a given length. However, the production of high shirring densities requires complicated shirring rollers. The individual shirring teeth of a roller have to have different shapes, which requires very expensive casting moulds and adds to the cost of the manufacturing process.
In principle, therefore a shirring device of which the shirring rollers have teeth of identical shape would be highly desirable. However, when using shirring rollers with identical teeth, the quality of the shirring depends very greatly on the ratio of the gut diameter to the diameter of the shirring mandrel. If the gut diameter is near the upper limit within the tolerances resulting from the production process, shirring with shirring rollers of identical teeth gives oval gut "caterpillars" which can be filled only with difficulty on modern filling machines.
To produce greater shirring density a further previous proposal comprises allowing the shirring mandrel or the shirring head to swivel about the longitudinal axis of the shirring mandrel. Whilst this achieves some degree of mutual displacement and anchoring of the shirred folds, the conventional shirred form, in which the individual shirred folds are not displaced relative to one another, is produced at the points of reversal of the swivelling motion. At these points, the stiffness of the caterpillar is less than at the other points.
It is an object of the present invention to provide a shirring device which permits the use of shirring rollers of simple shape and with identical teeth, whilst nevertheless achieving very uniform shirring of good quality and at a high shirring ratio, and to provide a process for shirring tubular films on such a device.